B-spline
B-spline is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.
- Task
Generate a B-spline curve with a list of 12 points and plot or save image.
Coordinates of control points:
start=171,171 1 185,111, 2 202,109, 3 202,189 4 328,160 5 208,254 6 241,330 7 164,252 8 69,278 9 139,208 10 72,148 end=168,172
Rules!!!!
Do not use third party libraries or functions
- See also
ALGOL 68
Which is
Suppresses unused parts of the plot.
BEGIN # construct a B-Spline #
# mode to hold a B Spline #
MODE BSPLINE = STRUCT( FLEX[ 1 : 0, 1 : 2 ]INT control points
, INT n
, INT k
, FLEX[ 1 : 0 ]INT t
);
PROC uniform knot vector = ( INT lb, ub )[]INT:
BEGIN
[ lb : ub ]INT range;
FOR n FROM lb TO ub DO range[ n ] := n OD;
range
END # uniform knot vector # ;
PROC calculate bspline = ( REF BSPLINE bs, INT i, k, x )REAL:
IF k = 1
THEN ABS ( ( t OF bs )[ i ] <= x AND x < ( t OF bs )[ i + 1 ] )
ELSE
PROC helper = ( REF BSPLINE bs, INT i, k, x )REAL:
IF ( t OF bs )[ i + k ] /= ( t OF bs )[ i ]
THEN ( x - ( t OF bs )[ i ] ) / ( ( t OF bs )[ i + k ] - ( t OF bs )[ i ] )
ELSE 0
FI # helper # ;
( helper( bs, i, k - 1, x ) ) * calculate bspline( bs, i, k - 1, x )
+ ( 1 - helper( bs, i + 1, k - 1, x ) ) * calculate bspline( bs, i + 1, k - 1, x )
FI # calculate bspline # ;
PROC round = ( REAL n )INT: ENTIER( n + 0.5 );
PROC get bspline points = ( REF BSPLINE bs )[,]INT:
BEGIN
INT p from = ( t OF bs )[ k OF bs ];
INT p to = ( t OF bs )[ 1 + n OF bs ] - 1;
[ p from : p to, 1 : 2 ]INT points;
FOR x FROM p from TO p to DO
REAL sum x := 0;
REAL sum y := 0;
FOR i TO n OF bs DO
REAL f = calculate bspline( bs, i, k OF bs, x );
sum x +:= f * ( control points OF bs )[ i, 1 ];
sum y +:= f * ( control points OF bs )[ i, 2 ]
OD;
points[ x, 1 ] := round( sum x );
points[ x, 2 ] := round( sum y )
OD;
points
END # get bspline points # ;
PROC raytrace = ( INT x0, y0, x2, y2, REF[,]BOOL plot, PROC( REF[,]BOOL, INT, INT )VOID visit )VOID:
BEGIN
INT x := x0;
INT y := y0;
INT dx := ABS ( x2 - x );
INT dy := ABS ( y2 - y );
INT n = 1 + dx + dy;
INT dir x = IF x2 > x THEN 1 ELSE -1 FI;
INT dir y = IF y2 > y THEN 1 ELSE -1 FI;
INT err := dx - dy;
dx *:= 2;
dy *:= 2;
FOR i TO n DO
visit( plot, x, y );
IF err > 0
THEN x +:= dir x; err -:= dy
ELSE y +:= dir y; err +:= dx
FI
OD
END # raytrace # ;
PROC plot line = ( REF[,]BOOL plot, INT x1, y1, x2, y2 )VOID:
raytrace( x1, y1, x2, y2, plot
, ( REF[,]BOOL plot, INT x, INT y )VOID: IF x >= 0
AND y >= 0
AND x < 1 UPB plot
AND y < 2 UPB plot
THEN plot[ x + 1, y + 1 ] := TRUE
FI
);
PROC plot bspline = ( REF BSPLINE bs, REF[,]BOOL plot, REAL scale x, scale y )VOID:
IF k OF bs > n OF bs OR k OF bs < 1 THEN
print( ( "k (= ", whole( k OF bs, 0 ), ") can't be more than ", whole( n OF bs, 0 ) ) );
print( ( " or less than 1.", newline ) );
stop
ELSE
[,]INT points = get bspline points( bs );
# Plot the curve. #
FOR i FROM 1 LWB points TO 1 UPB points - 1 DO
INT p1x = points[ i, 1 ], p1y = points[ i, 2 ];
INT p2x = points[ i + 1, 1 ], p2y = points[ i + 1, 2 ];
plot line( plot
, round( p1x * scale x ), round( p1y * scale y )
, round( p2x * scale x ), round( p2y * scale y )
)
OD
FI # plot bspline # ;
# print the plot - outputs @ or blank depending on whether the point is plotted or not #
PROC print plot = ( [,]BOOL plot )VOID:
FOR row FROM 1 LWB plot
TO BEGIN # find the highest used row #
INT max row := 1 UPB plot;
WHILE IF max row < 1 LWB plot
THEN FALSE
ELSE
BOOL empty row := TRUE;
FOR column FROM 2 LWB plot TO 2 UPB plot
WHILE empty row := NOT plot[ column, max row ]
DO
SKIP
OD;
empty row
FI
DO
max row -:= 1
OD;
max row
END
DO
INT max column := 2 UPB plot;
WHILE IF max column < 2 LWB plot THEN FALSE ELSE NOT plot[ max column, row ] FI
DO
max column -:= 1
OD;
FOR column FROM 2 LWB plot TO max column DO
print( ( IF plot[ column, row ] THEN "@" ELSE " " FI ) )
OD;
print( ( newline ) )
OD # print plot # ;
# task #
[,]INT control points
= ( ( 171, 171 ), ( 185, 111 ), ( 202, 109 ), ( 202, 189 ), ( 328, 160 ), ( 208, 254 )
, ( 241, 330 ), ( 164, 252 ), ( 69, 278 ), ( 139, 208 ), ( 72, 148 ), ( 168, 172 )
);
INT k = 4; # Polynomial degree is one less than this i.e. cubic. #
BSPLINE bs := ( control points
, UPB control points
, k
, uniform knot vector( 1, UPB control points + k )
);
REAL scale x = 0.4; # Since we print the plot to the console as text let's scale things appropriately. #
REAL scale y = 0.2;
[ 1 : 350, 1 : 350 ]BOOL plot;
FOR r FROM 1 LWB plot TO 1 UPB plot DO FOR c FROM 2 LWB plot TO 2 UPB plot DO plot[ c, r ] := FALSE OD OD;
plot bspline( bs, plot, scale x, scale y );
print plot( plot )
END- Output:
leading blank lines removed...
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Julia
Choose BSpline D of 2, ie degree 1.
using Graphics, Plots
Point(t::Tuple) = Vec2(Float64(t[1]), Float64(t[2]))
const controlpoints = Point.([(171, 171), (185, 111), (202, 109), (202, 189), (328, 160),
(208, 254), (241, 330), (164,252), (69, 278), (139, 208), (72, 148), (168, 172)])
plt = plot(map(a -> a.x, controlpoints), map(a -> a.y, controlpoints))
savefig(plt, "BSplineplot.png")
Lua
local function Range(from, to)
local range = {}
for n = from, to do table.insert(range, n) end
return range
end
local function Bspline(controlPoints, k)
return {
controlPoints = controlPoints,
n = #controlPoints,
k = k,
t = Range(1, #controlPoints+k), -- Use a uniform knot vector, delta=1.
}
end
local function helper(bspline, i, k, x)
return (bspline.t[i+k] ~= bspline.t[i])
and (x - bspline.t[i]) / (bspline.t[i+k] - bspline.t[i])
or 0
end
local function calculateBspline(bspline, i, k, x)
if k == 1 then
return (bspline.t[i] <= x and x < bspline.t[i+1]) and 1 or 0
end
return ( helper(bspline, i , k-1, x)) * calculateBspline(bspline, i , k-1, x)
+ (1-helper(bspline, i+1, k-1, x)) * calculateBspline(bspline, i+1, k-1, x)
end
local function round(n)
return math.floor(n+.5)
end
local function getBsplinePoints(bspline)
local points = {}
for x = bspline.t[bspline.k], bspline.t[bspline.n+1]-1 do
local sumX = 0
local sumY = 0
for i = 1, bspline.n do
local f = calculateBspline(bspline, i, bspline.k, x)
sumX = sumX + f * bspline.controlPoints[i].x
sumY = sumY + f * bspline.controlPoints[i].y
end
table.insert(points, {x=round(sumX), y=round(sumY)})
end
return points
end
local function Plot(unscaledWidth,unscaledHeight, scaleX,scaleY)
local plot = {
width = round(unscaledWidth * scaleX),
height = round(unscaledHeight * scaleY),
scaleX = scaleX,
scaleY = scaleY,
}
for row = 1, plot.height do
plot[row] = {}
end
return plot
end
local function raytrace(x,y, x2,y2, visit)
local dx = math.abs(x2 - x)
local dy = math.abs(y2 - y)
local n = 1 + dx + dy
local dirX = (x2 > x) and 1 or -1
local dirY = (y2 > y) and 1 or -1
local err = dx - dy
dx, dy = 2*dx, 2*dy
for n = 1, n do
visit(x, y)
if err > 0 then x, err = x+dirX, err-dy
else y, err = y+dirY, err+dx end
end
end
local function plotLine(plot, x1,y1, x2,y2)
raytrace(x1,y1, x2,y2, function(x, y)
if x >= 0 and y >= 0 and x < plot.width and y < plot.height then
plot[y+1][x+1] = true
end
end)
end
local function plotBspline(bspline, plot)
if bspline.k > bspline.n or bspline.k < 1 then
error("k (= "..bspline.k..") can't be more than "..bspline.n.." or less than 1.")
end
local points = getBsplinePoints(bspline)
-- Plot the curve.
for i = 1, #points-1 do
local p1 = points[i]
local p2 = points[i+1]
plotLine(plot,
round(p1.x*plot.scaleX), round(p1.y*plot.scaleY),
round(p2.x*plot.scaleX), round(p2.y*plot.scaleY)
)
end
end
local function printPlot(plot)
for row = 1, plot.height do
for column = 1, plot.width do
io.write(plot[row][column] and "@" or " ")
end
io.write("\n")
end
end
local controlPoints = {
{x=171, y=171}, {x=185, y=111}, {x=202, y=109}, {x=202, y=189}, {x=328, y=160}, {x=208, y=254},
{x=241, y=330}, {x=164, y=252}, {x= 69, y=278}, {x=139, y=208}, {x= 72, y=148}, {x=168, y=172},
}
local k = 4 -- Polynomial degree is one less than this i.e. cubic.
local bspline = Bspline(controlPoints, k)
local scaleX = .4 -- Since we print the plot to the console as text let's scale things appropriately.
local scaleY = .2
local plot = Plot(350,350, scaleX,scaleY)
plotBspline(bspline, plot)
printPlot(plot)
- Output:
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Mathematica/Wolfram Language
Graphics[
BSplineCurve[{{171, 171}, {185, 111}, {202, 109}, {202, 189}, {328,
160}, {208, 254}, {241, 330}, {164, 252}, {69, 278}, {139,
208}, {72, 148}, {168, 172}}, SplineClosed -> True,
SplineDegree -> 2]]
- Output:
Outputs a graphical representation of a B-spline.
Perl
use strict;
use warnings;
use Class::Struct;
use Cairo;
{ package Line;
struct( A => '@', B => '@');
}
my ($WIDTH, $HEIGHT, $W_LINE, $CURVE_F, $DETACHED, $OUTPUT ) =
( 400, 400, 2, 0.25, 0, 'run/b-spline.png' );
my @pt = (
[171, 171], [185, 111], [202, 109], [202, 189], [328, 160], [208, 254],
[241, 330], [164, 252], [ 69, 278], [139, 208], [ 72, 148], [168, 172]
);
my $cnt = @pt;
sub angle {
my($g) = @_;
atan2 $g->B->[1] - $g->A->[1], $g->B->[0] - $g->A->[0]
}
sub control_points {
my($g, $l) = @_;
my $h = Line->new;
my $lgt = sqrt( ($g->B->[0] - $l->A->[0])**2 + ($g->B->[1] - $l->A->[1])**2 );
@{$h->B} = @{$l->A};
@{$h->A} = ($g->B->[0] - $lgt * cos(angle $g) , $g->B->[1] - $lgt * sin(angle $g));
my $a = angle $h;
my @p1 = ($g->B->[0] + $lgt * cos($a) * $CURVE_F, $g->B->[1] + $lgt * sin($a) * $CURVE_F);
@{$h->A} = @{$g->B};
@{$h->B} = ($l->A->[0] + $lgt * cos(angle $l) , $l->A->[1] + $lgt * sin(angle $l));
$a = angle $h;
my @p2 = ($l->A->[0] - $lgt * cos($a) * $CURVE_F, $l->A->[1] - $lgt * sin($a) * $CURVE_F);
\@p1, \@p2
}
my $surf = Cairo::ImageSurface->create ('argb32', $WIDTH, $HEIGHT);
my $cr = Cairo::Context->create ($surf);
$cr->set_line_width($W_LINE);
$cr->move_to($pt[$DETACHED - 1 + $cnt][0], $pt[$DETACHED - 1 + $cnt][1]);
my Line ($g,$l);
for my $j ($DETACHED..$cnt-1) {
$g = Line->new( A=>$pt[($j + $cnt - 2) % $cnt], B=>$pt[($j + $cnt - 1) % $cnt]);
$l = Line->new( A=>$pt[($j + $cnt + 0) % $cnt], B=>$pt[($j + $cnt + 1) % $cnt]);
my($p1,$p2) = control_points($g, $l);
$cr->curve_to($$p1[0], $$p1[1], $$p2[0], $$p2[1], $pt[$j][0], $pt[$j][1]);
}
$cr->stroke;
$surf->write_to_png($OUTPUT);
Output: b-spline.png (offsite image)
Phix
You can run this online here.
-- -- demo\rosetta\B-spline.exw -- ========================= -- -- Use +/- to change the order between k = 1 and k = 4. -- with javascript_semantics include pGUI.e include IupGraph.e constant ctrl_points = {{171, 171}, {185, 111}, {202, 109}, {202, 189}, {328, 160}, {208, 254}, {241, 330}, {164, 252}, { 69, 278}, {139, 208}, { 72, 148}, {168, 172}} integer k = 2, n sequence t function w(integer i, k, x) // B-spline helper function return iff(t[i+k]!=t[i] ? (x-t[i])/(t[i+k]-t[i]) : 0 ) end function function b(integer i, k, x) // B-spline function if k==1 then return iff(t[i]<=x and x<t[i+1] ? 1 : 0) end if return w(i,k-1,x)*b(i,k-1,x) + (1-w(i+1,k-1,x))*b(i+1,k-1,x) end function function b_spline(Ihandle graph) n = length(ctrl_points) t = tagset(n+1+k) // use a uniform knot vector, delta = 1 assert(k<=n+1 and k>=1,"k (= %d) cannot be more than %d or less than 1.",{k,n+1}) sequence px = {}, py = {} for x=t[k] to t[n+1] do atom sumX = 0, sumY = 0 for i=1 to n do atom f = b(i,k,x) sumX += f*ctrl_points[i][1] sumY += f*ctrl_points[i][2] end for px &= round(sumX) py &= round(sumY) end for integer xtick = 40, ytick = 40, xmin = trunc(min(px)/xtick)*xtick, xmax = ceil(max(px)/xtick)*xtick, ymin = trunc(min(py)/ytick)*ytick, ymax = ceil(max(py)/ytick)*ytick IupSetInt(graph,"XTICK",xtick) IupSetInt(graph,"XMIN",xmin) IupSetInt(graph,"XMAX",xmax) IupSetInt(graph,"YTICK",ytick) IupSetInt(graph,"YMIN",ymin) IupSetInt(graph,"YMAX",ymax) sequence graphdata = {{px,py,CD_BLUE}} return graphdata end function procedure set_title(Ihandle dlg) IupSetStrAttribute(dlg, "TITLE", "B-spline curve (order k = %d)",{k}) end procedure function key_cb(Ihandle dlg, atom c) if c=K_ESC then return IUP_CLOSE end if if c='+' then k = min(k+1,4) end if if c='-' then k = max(k-1,1) end if set_title(dlg) IupRedraw(dlg) return IUP_IGNORE end function procedure main() IupOpen() Ihandle graph = IupGraph(b_spline,`RASTERSIZE=600x600`) Ihandle dlg = IupDialog(graph) IupSetCallback(dlg, "KEY_CB", Icallback("key_cb")) set_title(dlg) IupShow(dlg) if platform()!=JS then IupMainLoop() IupClose() end if end procedure main()
Processing
//Aamrun, 26th June 2022
int abscissae[]={171,185,202,202,328,208,241,164,69,139,72,168};
int ordinates[] = {171,111,109,189 ,160,254 ,330 ,252,278 ,208 ,148 ,172};
void setup() {
size(450, 450);
background(255);
smooth();
noFill();
stroke(0);
beginShape();
for(int i=0;i<abscissae.length;i++){
curveVertex(abscissae[i], ordinates[i]);
}
endShape();
fill(255, 0, 0);
noStroke();
for (int i = 0; i < abscissae.length; i ++) {
ellipse(abscissae[i], ordinates[i], 3, 3);
}
}
Raku
A minimal translation of this C program, by Bernhard R. Fischer.
# 20211112 Raku programming solution
use Cairo;
# class point_t { has Num ($.x,$.y) is rw } # get by with two element lists
class line_t { has ($.A,$.B) is rw }
my (\WIDTH, \HEIGHT, \W_LINE, \CURVE_F, \DETACHED, \OUTPUT ) =
400, 400, 2, 0.25, 0, './b-spline.png' ;
my \cnt = #`(Number of points) ( my \pt = [
[171, 171], [185, 111], [202, 109], [202, 189], [328, 160], [208, 254],
[241, 330], [164, 252], [ 69, 278], [139, 208], [ 72, 148], [168, 172], ]
).elems;
sub angle(\g) { atan2(g.B.[1] - g.A.[1], g.B.[0] - g.A.[0]) }
sub control_points(\g, \l, @p1, @p2){
#`[ This function calculates the control points. It takes two lines g and l as
* arguments but it takes three lines into account for calculation. This is
* line g (P0/P1), line h (P1/P2), and line l (P2/P3). The control points being
* calculated are actually those for the middle line h, this is from P1 to P2.
* Line g is the predecessor and line l the successor of line h.
* @param g Pointer to first line (P0 to P1)
* @param l Pointer to third line (P2 to P3)
* @param p1 Pointer to memory of first control point.
* @param p2 Pointer to memory of second control point. ]
my \h = $ = line_t.new;
my \lgt = sqrt([+]([ g.B.[0]-l.A.[0], g.B.[1]-l.A.[1] ]>>²));#length of P1 to P2
h.B = l.A.clone; # end point of 1st tangent
# start point of tangent at same distance as end point along 'g'
h.A = g.B.[0] - lgt * cos(angle g) , g.B.[1] - lgt * sin(angle g);
my $a = angle h ; # angle of tangent
# 1st control point on tangent at distance 'lgt * CURVE_F'
@p1 = g.B.[0] + lgt * cos($a) * CURVE_F, g.B.[1] + lgt * sin($a) * CURVE_F;
h.A = g.B.clone; # start point of 2nd tangent
# end point of tangent at same distance as start point along 'l'
h.B = l.A.[0] + lgt * cos(angle l) , l.A.[1] + lgt * sin(angle l);
$a = angle h; # angle of tangent
# 2nd control point on tangent at distance 'lgt * CURVE_F'
@p2 = l.A.[0] - lgt * cos($a) * CURVE_F, l.A.[1] - lgt * sin($a) * CURVE_F;
}
given Cairo::Image.create(Cairo::FORMAT_ARGB32, WIDTH, HEIGHT) {
given Cairo::Context.new($_) {
my line_t ($g,$l);
my (@p1,@p2);
.line_width = W_LINE;
.move_to(pt[DETACHED - 1 + cnt].[0], pt[DETACHED - 1 + cnt].[1]);
for DETACHED..^cnt -> \j {
$g = line_t.new: A=>pt[(j + cnt - 2) % cnt], B=>pt[(j + cnt - 1) % cnt];
$l = line_t.new: A=>pt[(j + cnt + 0) % cnt], B=>pt[(j + cnt + 1) % cnt];
# Calculate controls points for points pt[j-1] and pt[j].
control_points($g, $l, @p1, @p2);
.curve_to(@p1[0], @p1[1], @p2[0], @p2[1], pt[j].[0], pt[j].[1]);
}
.stroke;
};
.write_png(OUTPUT) and die # C return
}
Output: (Offsite image file)
Wren
In the absence of any clarification on what to use (see Talk page), the following uses a degree of 3 (i.e order k = 4) and a uniform knot vector from 1 to 16 (as there are 12 control points) with a delta of 1.
If one uses a value for k of 1, then the script will simply plot the control points as in the Julia example.
import "dome" for Window, Process
import "graphics" for Canvas, Color
class BSpline {
construct new(width, height, cpoints, k) {
Window.resize(width, height)
Canvas.resize(width, height)
Window.title = "B-spline curve"
_p = cpoints
_n = cpoints.count - 1
_k = k
_t = (1.._n + 1 + k).toList // use a uniform knot vector, delta = 1
}
// B-spline helper function
w(i, k, x) { (_t[i+k] != _t[i]) ? (x - _t[i]) / (_t[i+k] - _t[i]) : 0 }
// B-spline function
b(i, k, x) {
if (k == 1) return (_t[i] <= x && x < _t[i + 1]) ? 1 : 0
return w(i, k-1, x) * b(i, k-1, x) + (1 - w(i+1, k-1, x)) * b(i+1, k-1, x)
}
// B-spline points
p() {
var bpoints = []
for (x in _t[_k-1]..._t[_n + 1]) {
var sumX = 0
var sumY = 0
for (i in 0.._n) {
var f = b(i, _k, x)
sumX = sumX + f * _p[i][0]
sumY = sumY + f * _p[i][1]
}
bpoints.add([sumX.round, sumY.round])
}
return bpoints
}
init() {
if (_k > _n + 1 || _k < 1) {
System.print("k (= %(_k)) can't be more than %(_n+1) or less than 1.")
Process.exit()
}
var bpoints = p()
// plot the curve
for (i in 1...bpoints.count) {
Canvas.line(bpoints[i-1][0], bpoints[i-1][1], bpoints[i][0], bpoints[i][1], Color.white)
}
}
update() {}
draw(alpha) {}
}
var cpoints = [
[171, 171], [185, 111], [202, 109], [202, 189], [328, 160], [208, 254],
[241, 330], [164, 252], [ 69, 278], [139, 208], [ 72, 148], [168, 172]
]
var k = 4 // polynomial degree is one less than this i.e. cubic
var Game = BSpline.new(400, 400, cpoints, k)